Travertine versus Calcareous tufa: distinctive petrologic features and stable isotopes signatures

Abstract

A. Gandin & E. Capezzuoli, Travertine versus Calcareous tufa: distinctive petrologic features and stable isotopes signatures.
(IT ISSN 0394-3356, 2007).
Terrestrial limestones, mainly deposited as calcite crusts by carbonate-rich waters flowing in subaerial settings, originate from warm- to-hot waters of hydrothermal provenance (travertines) or from cool waters of karstic derivation (calcareous tufa). The two groups of
concretionary carbonates are actively forming in different geomorphologic settings such as cool-water springs and fluvial valleys, or in
the surroundings of the emergence of thermal springs. Fossil remains of these rocks are frequently found in calcareous areas as well as in areas where carbonate rocks are not exposed. However the recognition on the field of their genetic derivation when connections
with the feeding source are no more evident, is often uncertain since the definition of the specific litofacies are not yet completely elu- cidated.
The detailed analysis of the lithofacies of calcareous tufa and travertines as well as the critical elaboration of existing geochemical
data, shows that the petrologic features and stable isotopic signatures reflect the contrasting environmental conditions of deposition
deriving from physico-chemical properties of the feeding waters that, although mostly of meteoric provenance, experienced different
circulation/recharge history.
Travertines are well bedded, often finely laminated compact limestones, composed in the proximal part of the thermal system by
peculiar crystalline crusts and in the distal part by bacterial/cyanobacterial laminites often associated with thin crystalline crusts, len- ses of calcified-bubbles and paper-thin rafts.
In this system the carbonate deposition mainly derives from intense outgassing of the emerging supersaturated waters, and evapora- tion, concomitant with the rapid drop in temperature along the drainage network.
Travertine mineral content originates in deep geothermal/hydrothermal conditions where hot waters charged with HCO3
- of meteoric and hypogean derivation, are able to dissolve high quantities of carbonate and/or evaporitic bedrock. The circulation of hot waters, allowed by open fractures/faults is ultimately controlled by extensional tectonics.
Calcareous tufa are poorly bedded, porous/chalky deposits containing frequent remains of macrophytes and invertebrates. They are
made up of dominantly microcrystalline calcite of abiotic and biotic precipitation, forming phytohermal, stromatolitic or phytoclastic
facies. It is commonly found in fluvial/palustrine depositional systems fed by carbonate-rich waters mostly deriving from karstic springs. The carbonate deposition deriving from mainly mechanical (vaporization/evaporation) and less significant biologic removal of
CO2 from flowing or standing waters, results to be controlled by the availability of meteoric water and ultimately by local and/or global
climatic conditions.
The comparison of petrofacies and the critical elaboration of the related stable isotopic signatures of deposits still connected with the
parent water and source systems, provide criteria for the univocal lithologic identification of their fossil counterparts so that the mainly
climate-controlled Calcareous Tufa and the mainly tectonics-controlled Travertines can be easily detected even on the field.